The guidelines, [http://www.copyright.gov/circs/circ07b.pdf ‘Best Edition’ of Published Copyrighted Works for the Collections of the Library of Congress,] were written for a particular case, but they might be useful in choosing which of two copies to keep in the absence of other criteria in your library.

However, before you seek a destination for your discards, do have a look at the table on the third page of Dickinson, Gail. "Crying Over Spilled Milk (PDF)." Library Media Connection. 23, no. 7 (2005): 24-26.

Periodicals

ALCTS Duplicates Exchange Union - ALCTS is the Association for Library Collections & Technical Services, a division of ALA. DEU is an autonomous group of libraries that communicate via electronic discussion list to exchange usable library material. Membership is free and open to all libraries.